Field of the Invention
The present invention relates to a method, computer method, system, and apparatus for measuring concentrations of chemical species in semiconductor plasma processing using plasma optical emission spectroscopy (OES). Specifically, it relates to determining two-dimensional distributions of plasma optical emissions from which two-dimensional distributions of chemical species concentrations can be determined.
Description of Related Art
Production of semiconductor devices, displays, photovoltaics, etc., proceeds in a sequence of steps, each step having parameters optimized for maximum device yield. In plasma processing, among the controlled parameters strongly affecting yield is the chemistry of the plasma, and particularly the local chemistry of the plasma, i.e. the local concentrations of various chemical species in the plasma environment proximate the substrate being processed. Certain species, particularly transient chemical species, such as radicals have a great influence on the plasma processing outcome, and it is known that elevated local concentrations of these species can produce areas of faster processing, which may lead to nonuniformities in the processing steps and ultimately the devices being produced.
The chemistry of a plasma process is controlled in a direct or indirect manner through the control of a large number of process variables, such as one or more RF or microwave powers supplied to excite the plasma, the gas flows and kinds of gases supplied to the plasma processing chamber, the pressure in the plasma processing chamber, the type of substrate being processed, the pumping speed delivered to the plasma processing chamber, and many more. Optical emission spectroscopy (OES) has proven itself as a useful tool for process development and monitoring in plasma processing. In optical emission spectroscopy, the presence and concentrations of certain chemical species of particular interest, such as radicals, is deduced from acquired optical (i.e. light) emission spectra of the plasma, wherein the intensities of certain spectral lines and ratios thereof correlate to the concentrations of chemical species. A detailed description of the technique can be found in e.g. G. Selwyn, “Optical Diagnostic Techniques for Plasma Processing”, AVS Press, 1993, and will not be repeated here, for brevity.
While the use of optical emission spectroscopy has become relatively commonplace, particularly in plasma process development, it is usually done by acquiring optical emission spectra from a single elongated volume within the plasma, inside the plasma processing chamber. The precise shape and size of this volume is determined by the optical system used to collect the optical emission from the plasma. Such collection of the optical emission signal inherently results in averaging of the plasma optical emission spectra along the length of this elongated volume, also known as a ray, and thus all the information about local variations of the plasma optical emission spectra, and thus also local variations of chemical species concentrations, are generally lost.
In development of plasma processes, and indeed even in development of new and improved plasma processing systems, it is useful to know the two-dimensional distribution of chemical species of interest above the substrate being processed, so changes in the system design and/or process parameters can be made to minimize variations of the processing outcome across the substrate, for example. A further application of the plasma optical emission spectroscopy (OES) technique is in determining the endpoint of a plasma processing step by monitoring the evolution of and abrupt change of chemical species present in the plasma that is associated with e.g. an etching step reaching a substrate layer of different chemical composition that the one that was etched during the etching process. The ability to determine the plasma processing step endpoint across the entire surface of the substrate contributes to increased device yield because of not terminating the plasma processing step prematurely.
One technique extensively used in other areas of technology, e.g. X-ray tomography, to determine a spatial distribution of a variable from known integrated measurements along multiple rays traversing the area of interest is tomographic inversion, using the Abel transform, or Radon transform. However, to be effective, this technique requires a large amount of acquired data, i.e. a large number of rays, which is impractical in a semiconductor processing tool that has limited optical access to the plasma through one or a small number of windows or optical ports built into the plasma processing chamber wall. Tomographic techniques are generally also very computationally intensive. It has also been found that local variations of chemical species concentrations are of a generally smooth nature, without any abrupt gradients in both the radial, and even more so in the circumferential (i.e. azimuthal) direction. Thus, it would be advantageous to have a simple, fast, and relatively low cost plasma optical emission spectroscopy (OES) technique and system that is capable of acquiring the two-dimensional distributions of plasma optical emission spectra without the overhead involved in tomographic approaches to OES measurements.
Most notably, while the variations in the circumferential direction may be small, they are not nonexistent, as some prior techniques presume, and the ideal technique and system would still have to be able to reliably capture these variations.